Insulating a wellbore in permafrost

ABSTRACT

A method and apparatus for maintaining at least a portion of the interior region of a wellbore thermally insulated from the wall of the wellbore wherein a single zone in the wellbore is utilized for the introduction, circulation, and return of cooling fluid.

United States Patent Keeler et al.

[54] INSULATING A WELLBORE IN PERMAFROST [72] Inventors: William A.Keeler; Frank J. Schuh, both of Dallas, Tex.

[73] Assignee: Atlantic Richiield Company, New York,

[22] Filed: Apr. 30, 1970 [21] Appl.No.: 33,223

15] 3,662,832 [4 1 May 16, 1972 3,227,215 1/1966 Marx ....l66/53 50,39910/1865 1,342,780 6/1920 3,004,601 10/1961 3,013,609 12/1961 3,217,79111/1965 Long ..l66/D1G.l

OTHER PUBLICATIONS Alaskan Completions Will be Complicated. in WorldOil, Jan. 1970, p. 85.

Primary ExaminerDavid H. Brown Attorne'y-Blucher S. Tharp and RoderickW, MacDonald [5 7] ABSTRACT A method and apparatus for maintaining atleast a portion of the interior region of a wellbore thermally insulatedfrom the wall of the wellbore wherein a single zone in the wellbore isutilized for the introduction, circulation, and return of cooling fluid.

9 Claims, 3 Drawing Figures IIB PATENTEDMAY 16 I972 SHEET 1 BF 2 I FIG.I

ATTGRMEY BACKGROUND OF THE INVENTION In Northern Alaska, Canada, andother frigid areas of the world, a larger and larger number of oiland/or gas wells are being drilled through permafrost. It is recognizedthat the production of relatively warm fluids through the wellbore overthe production life of a well could cause melting of the permafrost andthis in turn could cause failure of the casing and, therefore, the well.

Accordingly, various proposals have been made for protecting thepermafrost adjacent the wellbore from being thawed by warm fluidsproduced through the wellbore.

One such proposal is fully and completely disclosed in World Oil, Jan.1970, page 85, the disclosure of which is incorporated herein byreference. This proposal utilizes a 20-inch diameter outer casing and a13 36 inch diameter inner casing to establish an area which is splitinto two zones by inserting a 16-inch casing between the 13 as inch and20-inch casings. Thus, two cooling zones are established, one on eitherside of the 16-inch casing, and liquid refrigerant is pumped down intothe wellbore in one cooling zone annulus and is returned, as warmliquid, up the other cooling zone annulus to the earths surface.

SUMMARY OF THE INVENTION According to this invention, cooling of thewellbore in the area of the permafrost is obtained by a method andapparatus employing only a single cooling zon By this invention a singleannulus for handling all of the introduction, circulation, and removalof cooling fluid from the wellbore is achieved, for example, by usingonly 13 l-inch and 9 %-inch casing. This invention, therefore,eliminates the need for the 16-inch casing of the World Oil referenceand further eliminates the need for using the 20-inch casing for theouter wall of the cooling zone.

Accordingly, by this invention substantial economic savings are enjoyednot only be eliminating the 16-inch casing without eliminating thefunction thereof, but also by allowing the use of a narrower diameterwellbore since the outer wall of the cooling zone can be 13 lb-inchcasing instead of the 20-inch casing of the World Oil reference.

This invention provides apparatus for thermally insulating a region in awellbore utilizing a single annulus means for transporting cooling fluidinto and out of the wellbore, means for introducing and removing coolingfluid from the single annulus means, and cooling means for cooling thecooling fluid after it has been removed from the single annulus means.

There is also provided a method of thermally insulating a region in awellbore wherein the single annulus means (single cooling zone) hasadded thereto a substantially liquid cooling fluid which will vaporizeat or below the maximum pressure in the single cooling zone, vaporizingthe cooling fluid by allowing same to absorb heat from the region to becooled, recovering the vaporized cooling fluid, and condensing thevaporized cooling fluid for return to the cooling zone.

Accordingly, it is an object of this invention to provide a new andimproved method and apparatus for producing a well in a permafrost areawithout substantial thawing of the permafrost. It is another object toprovide a new and improved method and apparatus for cooling at least aportion of a wellbore. It is another object to provide a new andimproved method and apparatus which will cool a wellbore withoutrequiring the use of a larger diameter wellbore and/or the use of anadditional casing string to establish separate annuli to handle theintroduction, circulation, and recovery of refrigerant. It is anotherobject to provide a new and improved method and apparatus which willrender the wellbore cooling system automatically self-refluxing.

Other aspects, objects, and advantages of this invention will beapparent to those skilled in the art from this disclosure and theappended claims.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. I shows a portion of awellbore which embodies one form of the single annulus cooling zoneconcept of this invention.

FIG. 2 shows surface cooling apparatus that can be used with theapparatus of FIG. 1.

FIG. 3 shows an alternative embodiment of surface cooling apparatus thatcan be employed with the apparatus of FIG. 1.

More specifically, FIG. 1 ,shows a wellbore wall 1 which has inwardlyextending steps 2 and 3 as smaller diameter casing is employed.

For example, casing 4, which can be 20-inch diameter casing, extends tostep 2 and any void between the outer surface of easing 4 and wellborewall 1 is filled in a conventional manner with cement 5. Casing 6, whichcan be 13 %-inch diameter casing, is extended downward to step 3 and thespace between the outer surface of easing 6 and wellbore wall 1 isfilled with cement 7 up to and slightly above the lower end of easing 4.Casing 8, which can be 9 Ks-inch diameter casing, is set in the wellborein a conventional manner with cement 9.

In the interior of easing 8 is disposed a tubing string 10 which can be,for example, 7-inch diameter tubing. Tubing l0 rests on packer 12 andprovides an opening 13 through which oil, gas, and other fluid isproduced from the bottom of the well up to the earth's surface.

In the annulus between tubing 10 and easing 8 is provided thermalinsulation material 11 such as polyurethane foam and the like.

Annulus 14 between casings 8 and 6 is the single annulus means (singlecooling zone) of this invention.

Annulus 15 between casings 6 and 4 can be filled with a packer fluidwhich itself can act as an insulating material Annulus 14 is notsubdivided into two separate annuli (represented by dotted line 18) inthe manner suggested by the World Oil reference. Rather, annulus 14 isleft open so that it is a single annulus or zone in which descending andrising cooling fluid circulates at will between casings 8 and 6. In theprior art, an additional casing string 18 established two separateannuli or cooling zones in which liquid refrigerant was circulated onlydownwardly in one, arrow 16, and only upwardly in the other, arrow 17.The warmed but still liquid refrigerant was then recovered at the earthssurface and cooled for reintroduction into only one of the two separatecooling zones.

Annulus 14 can have insulation 11 extend the full length thereof or havethe lower uninsulated portion contain a liquid 19 such as drilling fluidup to and preferably overlapping the lower end of insulation 1 I.

By the method of this invention, liquid cooling fluid is introduced intosingle cooling zone 14, allowed to travel freely down that zone, absorbheat from the region on the interior of easing 8 and tubing 10 whichcarries the relatively warm oil, gas and the like to the earth'ssurface, vaporize due to this heat absorption, and rise as a gascountercurrently with the downcoming liquid cooling fluid. The vaporizedcooling fluid is removed at the earth's surface, condensed, cooled, andreturned to zone 14.

FIG. 2 shows only zone 14 for sake of simplicity. Vaporized coolingfluid is removed from zone 14 through conduit means 20 to compressormeans 21. Compressor 21 compresses the vaporized cooling fluid to apressure substantially above its recovery pressure from zone 14, e.g.the pressure in recovery conduit 20 adjacent casing 6.

The compressed cooling fluid then passes by way of conduit means 22 toan ambient air fan cooler 23 which effects an indirect heat exchangerelationship between the compressed cooling fluid and ambient air tocool the compressed cooling fluid to a temperature approaching that ofthe ambient air temperature. This substantially cools the compressedcooling fluid for return by way of conduit means 24 through backpressure valve 27 to zone 14. Thus, gaseous cooling fluid Inna,

passes in the direction of arrow 25 while liquid cooling fluid passes inthe direction of arrow 26.

As an alternative to or an addition to ambient air fan cooler 23, anartificially refrigerated heat exhanger can be used to primarily orsupplementarily cool the cooling liquid that is to be returned to zone14. The artificially refrigerated heat exhanger can use any conventionalrefrigeration system, eg a separate refrigerant which is passed inindirect heat exchange relationship with the cooling fluid, the separaterefrigerant being separately compressed, cooled, and expanded through aconventional expansion valve in the well-known refrigeration cycle.

FIG. 3 shows cooling zone 14 by itself, for sake of simplicity, withcooling apparatus modified so as to render the system automaticallyself-refluxing. In this system conduits 30 and 32 slope upwardly fromzone 14 to cooling means 31.

By utilizing the apparatus of FIG. 3, vaporized cooling fluid risesprimarily in conduit 32 as shown by arrow 36 toward cooling means 31,although some vapor also rises through conduit 30. Due to the coolingeffect encountered in conduit 32, some of the vaporized cooling fluid iscondensed in conduit 32 and gravity feeds, without the use of anexternal pump, back in the direction of arrow 37 and into zone 14.

The vaporized cooling fluid which does reach cooling means 31 iscondensed and cooled and gravity feeds back to zone 14 by way ofdownwardly inclined conduit 30.

The system of FIG. 3 can be utilized during periods when the ambient airtemperature is relatively low, e.g. less than about F. When the ambientair temperature exceeds about 20 F it can be helpful to obtain greatercondensation and cooling of the cooling fluid to compress same before itreaches cooling means 31. This can be accomplished by an optional use ofconduit 33 for passing at least part of the gaseous fluid thereinthrough compressor 34 and then by way of conduit 35 back to conduit 32and on to cooling means 31.

As with cooling means 23 of FIG. 2, cooling means 31 can be any coolingmeans or a combination of cooling means such as an ambient air fancooler or an artificially refrigerated heat exchanger, and the like.

In the method of this invention, single cooling zone 14 is providedaround at least a portion of the region to be cooled and is alsodisposed intermediate the region to be cooled and the wellbore wall.Thereafter, substantially liquid cooling fluid which will vaporize at orbelow the maximum pressure in the cooling zone is added to that zone.The liquid cooling fluid is allowed to remain in the cooling zone untilit absorbs sufficient heat to vaporize same. The vaporized cooling fluidrises to the top of the cooling zone and is recovered at a recoverypressure, i.e. the pressure in conduit means 20 of FIG. 2 adjacent tocasing 6. The vaporized cooling fluid is then condensed by cooling or acombination of pressurization and cooling and thereafter is returned ascool liquid to the cooling zone.

If the vaporized cooling fluid is cooled sufficiently by an artificialrefrigeration system or the use of quite cold ambient air, e.g. lessthan 20 F. ambient air, sufficient condensation and cooling can beachieved without compression. However, if relatively stringent coolingis not achieved, e.g. when only ambient air is used and the ambient airtemperature is at least about 70 F compression of the vaporized coolingfluid to a pressure substantially above that of its recovery pressurecan be employed before heat exchange with ambient air. This achievesgreater condensation and cooling of the cooling liquid for return to thecooling zone.

Generally, any cooling fluid can be employed so long as the fluid can bemade to vaporize at less than 32 F. under pressures of less than 1,000psia. The cooling fluid is preferably a hydrocarbon or a halogenatedhydrocarbon, each having from one to eight, inclusive, carbon atoms permolecule or mixtures of two or more of such hydrocarbons and/orhalogenated hydrocarbons. The hydrocarbons are preferably straightorbranch-chained, saturated or unsaturated materials and the halogenatedhydrocarbons can be mono or polyhalogenated with one or more ofchlorine, bromine, iodine, fluorine, or combinations of two or more suchelements. Suitable specific materials include natural gas, methane,ethane, propane, butane, pentane, mixtures of two or more of thesematerials,

dichloromethane, difluoromethane, CO N11 methyl chloride, isobutane, S0dichlorotetrafluoroethane, dichloromonofluoromethane, ethyl chloride,

trichloromonofluoromethane, methyl formate, methylene chloride,trichlorotrifluoroethane, dichloroethylene, trichloroethylene, and thelike.

When the vaporized cooling fluid is compressed, it should be compressedto a pressure of at least about 50 psia greater than its recoverypressure above mentioned. Generally, the recovery pressure is less thanabout 400 psia and the vaporized cooling fluid is then compressed to apressure of at least about 450 psia. Preferably, the recovery pressureis from about to about 400 psia and is compressed to a pressure of fromabout 450 to about 1,000 psia.

The amount of cooling carried out on the vaporized cooling fluid,compressed or uncompressed, is that which is sufficient to substantiallycondense same to a liquid and cool same to a temperature below 32 F. forreintroduction into zone 14. The degree of cooling before re-entry intozone 14 can vary widely as desired, depending upon the cooling meansemployed. Generally, this temperature will be in a range of from about25 F. to no greater than 30 F.

It should be noted that by varying the recovery pressure, the amount ofliquid cooling fluid that can be tolerated in zone 14 and vaporizationof the cooling fluid still obtained can be varied. Thus, at a lowerrecovery pressure more liquid cooling fluid can be introduced into zone14 and better heat transfer obtained in zone 14 since the zone is closerto liquid full. However, with this lower recovery pressure, the coolingfluid has a lower maximum temperature of reintroduction into zone 14.With various cooling fluids this lower maximum pressure can besubstantially below 0 F. and can even approach the brittle temperaturefor the steel from which casings 8 and 6 are formed. Further, it is notdesirable to super-cool permafrost because there may be free water inpermafrost that can be subject to freezing.

Thus, too cold a cooling fluid might be damaging just as well as toowarm a cooling fluid so that it is preferable in most cases to maintainthe recovery pressure so that the cooling fluid is in the range of fromabout -25 F. to about 30 F. as it re-enters zone 14. It should also benoted that the amount of liquid cooling fluid present in the zone 14should be limited so that the pressure at the bottom of that coolingzone is not so great that the liquid cooling fluid will not vaporizebelow 32 F.

It should also be noted that the apparatus of FIGS. 2 and 3 can beoperated in accordance with the method of this invention in a mannersuch that the compression step is employed only when this is necessaryto cause liquefaction of the vaporized cooling fluid at the prevailingambient air temperature and that the compression step can be eliminatedat any time when the prevailing ambient air temperature is sufficientlylow to achieve a liquefaction and cooling of the cooling fluid into thetemperature range above described.

For example, when ethane is used as a cooling fluid in the apparatus ofFIG. 3 and the vaporized ethane recovery pressure in conduit 32 is 250psia, automatic self-refluxing of liquid ethane in conduit 32 isachieved. Adequate condensation and cooling of the remaining vapor isthen achieved by cooling means 31 when the prevailing ambient airtemperature is at least 10 F. below the 8 F. boiling point of liquidethane. In this situation, the use of compressor 34 would be eliminatedand only cooling means 31 operated as an ambient air fan cooler, liquidethane being returned to zone 14 by way of conduit 32 due tocondensation by cooling in that conduit and also by way of conduit 30due to cooling means 31.

Thus, in the embodiment of FIG. 3, liquid cooling fluid is returned tothe cooling zone at two points, i.e. the connection points of conduits32 and 30, rather than the one point, e.g.

the connection point of conduit 24 in FIG. 2. The return of liquidcooling fluid at two points is advantageous in that larger surface areasof casings 8 and 6 are contacted with liquid cooling fluid than when thecooling fluid is introduced at one point.

EXAMPLE The method of this invention is carried out in apparatussubstantially the same as that shown in FIGS. 1 and 2 using casing ofthe diameter dimensions set forth hereinabove with respect to theinitial description ofFlG. l.

Ethane is used as the cooling fluid and its recovery pressure in conduitis 150 psia. At this pressure the maximum allowable liquid ethane chargeto zone 14, so that the ethane at the bottom of the zone will stillvaporize at less than 32 F is 9,400 pounds. This gives a temperaturerange of maximum ethane charge to zone 14 of from to F.

Taking a prevailing ambient air temperature of 70 F compressor 21 (at acapacity of 1,500 pounds/hour) compresses vaporized ethane in conduit 20from the 150 psia recovery pressure to 700 psia. The compressed ethanethen passes through a conventional fin-fan cooler 23 where it is broughtinto indirect heat exchange relationship with the ambient air by meansof a fan and radiating fins. The compressed ethane is thereby cooledinto the temperature range of 25 to 30 F. The now liquefied and cooledethane is then returned by way of conduit 24 to zone 14.

By way of comparison, if in the above example the recovery pressure was250 psia instead of 150 psia, the maximum allowable liquid ethane chargein zone 14 would be limited to 4,700 pounds, a 4,700 pound reduction inorder to achieve vaporization of the liquid ethane at the bottom of zone14 at less than 32 F. Also, the temperature range of maximum ethanecharge would then change from the range of 25 to 30 F. to the range of 8to 30 F.

Reasonable variations and modifications are possible within the scope ofthis disclosure without departing from the spirit and scope of thisinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a method for thermally insulating a region in a wellbore from atleast a portion of the wall of the wellbore, said portion of the wall ofthe wellbore being in permafrost, the improvement comprising providing asingle annular cooling zone around at least a portion of said region,said single cooling zone being intermediate said region and saidwellbore wall, adding to said single cooling zone a substantially liquidcooling fluid which will vaporize at or below the maximum pressure insaid single cooling zone, vaporizing said liquid cooling fluid in saidsingle cooling zone without the use of an expansion device, saidvaporization being by absorption of heat from said region therebypreventing said heat from reaching and thawing said permafrost,recovering from said single cooling zone substantially vaporized coolingfluid, condensing said vaporized cooling fluid, returning condensedliquid cooling fluid to said single cooling zone, controlling therecovery pressure of said substantially vaporized cooling fluid tocontrol the vaporization of liquid cooling fluid in said cooling zone,and controlling the amount of liquid cooling fluid in said cooling zoneso that the pressure at the bottom of said cooling zone is not so greatthat said liquid cooling fluid will not vaporize below 32 F.

2. A method according to claim 1 wherein said cooling fluid vaporizes atless than 32 F. under a pressure of less than 1,000 psia.

3. A method according to claim 1 wherein said vaporized cooling fluid iscondensed by cooling same, said cooling being effected by contactingsaid vaporized cooling fluid in a heat exchange relationship without atleast one of ambient temperature air and artificially cooledrefrigerant.

4. A method according to claim 3 wherein said condensed cooling fluidgravity feeds without external pumping from the heat exchange zone tosaid singlepoolin zone.

5. A method according to claim 3 w eretn before cooling said vaporizedcooling fluid said fluid is first compressed to a pressure substantiallygreater than its recovery pressure so that said fluid will more readilycondense at the ambient air temperature.

6. A method according to claim 5 wherein said cooling fluid is ahydrocarbon having from one to eight, inclusive, carbon atoms permolecule, a monoor poly-halogenated hydrocarbon having from one toeight, inclusive, carbon atoms per molecule, a mixture of two or more ofsaid hydrocarbons and/or halogenated hydrocarbons, natural gas, CO S0and 7. A method according to claim 5 wherein said recovery pressure isless than about 400 psia and said fluid is compressed to a pressure ofat least about 450 psia.

8. A method according to claim 5 wherein said cooling fluid is a mixtureconsisting essentially of methane, ethane, propane, butane, and pentane,said recovery pressure is from about to about 400 psia, and said fluidis compressed to a pressure of from about 450 to about 1,000 psia.

9. A method according to claim 5 wherein said vaporized cooling fluid iscooled by heat exchange contact with ambient temperature air, and saidfluid is first compressed only when the temperature of the ambient airexceeds about 20 F.

1. In a method for thermally insulating a region in a wellbore from atleast a portion of the wall of the wellbore, said portion of the wall ofthe wellbore being in permafrost, the improvement comprising providing asingle annular cooling zone around at least a portion of said region,said single cooling zone being intermediate said region and saidwellbore wall, adding to said single cooling zone a substantially liquidcooling fluid which will vaporize at or below the maximum pressure insaid single cooling zone, vaporizing said liquid cooling fluid in saidsingle cooling zone without the use of an expansion device, saidvaporization being by absorption of heat from said region therebypreventing said heat from reaching and thawing said permafrost,recovering from said single cooling zone substantially vaporized coolingfluid, condensing said vaporized cooling fluid, returning condensedliquid cooling fluid to said single cooling zone, controlling therecovery pressure of said substantially vaporized cooling fluid tocontrol the vaporization of liquid cooling fluid in said cooling zone,and controlling the amount of liquid cooling fluid in said cooling zoneso that the pressure at the bottom of said cooling zone is not so greatthat said liquid cooling fluid will not vaporize below 32* F.
 2. Amethod according to claim 1 wherein said cooling fluid vaporizes at lessthan 32* F. under a pressure of less than 1,000 psia.
 3. A methodaccording to claim 1 wherein said vaporized cooling fluid is condensedby cooling same, said cooling being effected by contacting saidvaporized cooling fluid in a heat exchange relationship without at leastone of ambient temperature air and artificially cooled refrigerant.
 4. Amethod according to claim 3 wherein said condensed cooling fluid gravityfeeds without external pumping from the heat exchange zone to saidsingle cooling zone.
 5. A method according to claim 3 wherein beforecooling said vaporized cooling fluid said fluid is first compressed to apressure substantially greater than its recovery pressure so that saidfluid will more readily condense at the ambient air temperature.
 6. Amethod according to claim 5 wherein said cooling fluid is a hydrocarbonhaving from one to eight, inclusive, carbon atoms per molecule, a mono-or poly-halogenated hydrocarbon having from one to eight, inclusive,carbon atoms per molecule, a mixture of two or more of said hydrocarbonsand/or halogenated hydrocarbons, natural gas, CO2, SO2, and NH3.
 7. Amethod according to claim 5 wherein said recovery pressure is less thanabout 400 psia and said fluid is compressed to a pressure of at leastabout 450 psia.
 8. A method according to claim 5 wherein said coolingfluid is a mixture consisting essentially of methane, ethane, propane,butane, and pentane, said recovery pressure is from about 100 to about400 psia, and said fluid is compressed to a pressure of from about 450to about 1,000 psia.
 9. A method according to claim 5 wherein saidvaporized cooling fluid is cooled by heat exchange contact with ambienttemperature air, and said fluid is first compressed only when thetemperature of the ambient air exceeds about 20* F.